Something has bothered me about cycling infrastructure design for some time: when designing roads additional width is provided on tight corners to accommodate longer vehicles. This is needed because at low speed on tight bends the back wheels track inside the front wheel tracks - the longer the vehicle and the tighter the bend, the more width is required to ensure the rear wheels don't fall off the edge of the road.
I was reminded of this on Friday on my ride home where a misjudged a very tight corner on a narrow entry to the main pathway along the base of the Kangaroo Point cliffs. I almost took a tumble when my rear wheel went off the edge of the path and hit a hard kerb. It made me think, if I have difficulty getting around that bend, how would a longer or wider bicycle cope, like a cargo bike or a bike and trailer.
The Sustrans Design Manual provides guidance on the minimum turning circle for the front and rear wheels for different bikes.
What is very clear is that a longer bike like a bicycle and trailer would need to swing out further to make a tight corner that a regular bike could navigate easily. Since most of our paths have relatively low volumes, this should not present too much of a challenge. However as usage increases the need to provide extra width for turning bicycles will become more significant.
On bends with larger radii the issue of the tracking of the wheels becomes less significant. However at high speeds a turn requires cyclists to take more width of path than on a straight. Although this increased width is relatively small, the risk can increase significantly where there are high volumes of users in both directions. This can become a problem where the path has handrails, effectively forcing bicycle riders further away from the edge of the path to avoid the handrail instead of being able to lean outside of the path edge.
The V1 bikeway overpass of Stanley has relatively tight high-speed bends. In 2018 the pathway was upgraded and new handrails were provided (see the photo below) that reduce the effective encroachment of the handrail on the available space for bicycle riders turning the corner. My experience is that the new guardrails improves my feeling of safety in navigating the bend.
Is there a need for our cycling infrastructure design guidelines to recognise the needs of longer bicycles when designing corner radii? Should our designs recognise the additional space needed for cornering at speed, especially when the path has handrails adjacent?